Controlling parameters for radical cation fragmentation reactions: Origin of the intrinsic barrier

744

Controlling parameters for radical cation

fragmentation reactions: Origin of the intrinsic

barrier

Deepak Shukla, Guanghua Liu, Joseph P. Dinnocenzo, and Samir Farid

Abstract: C—C bond cleavages of radical cations of 2-substituted benzothiazoline derivatives were investigated to de-

termine the parameters controlling the fragmentation rate constants. In spite of the low oxidation potentials of the com-

pounds, fragmentation rate constants greater than 1 × 10⁶s^–1could be achieved through weakening of the fragmenting

bond by substituents that stabilize the radical fragment and exert steric crowding. A quantitative assessment of the rela-

tive roles of radical stabilization vs. steric effects to weaken the fragmenting C—C bond was achieved through DFT

calculations. The calculated activation enthalpies matched reasonably well with the experimentally determined values. A

thermokinetic analysis revealed that the fragmentations of benzothiazoline radical cations have relatively large intrinsic

kinetic barriers, ascribed to the delocalized nature of the product radical and cation fragments. Interestingly, the same

factors that lead to the large intrinsic barriers led, simultaneously, to large thermodynamic driving forces for the frag-

mentations, which should lead to lower activation barriers. These effects oppose each other kinetically and provide im-

portant insight into the design of fast radical ion fragmentation reactions.

Key words: benzothiazoline, radical cation, fragmentation, steric effects, DFT.

Résumé : Dans le but de déterminer les paramètres qui contrôlent la constante de vitesse de fragmentation, on a étudié

la rupture de la liaison C—C des cations radicalaires des dérivés de la benzothiazoline substituée en position 2. En dé-

pit du faible potentiel d’oxydation de ces composés, on peut obtenir des constantes de vitesse de fragmentation supé-

rieures à 1 × 10⁶s^–1en affaiblissant la liaison qui subit la fragmentation à l’aide de substituants qui stabilisent le

fragment radicalaire tout en exerçant un encombrement stérique. Faisant appel aux calculs DFT, on a évalué quantitati-

vement le rôle relatif de la stabilisation du radical par rapport aux effets stériques dans l’affaiblissement de la liaison

C—C qui se fragmente. Les enthalpies de liaisons calculées correspondent assez bien aux valeurs expérimentales. Une

analyse thermocinétique a révélé que les fragmentations des cations radicalaires de la benzothiazoline ont une large

barrière cinétique intrinsèque relative, attribuable à une délocalisation du radical et des fragments de cations obtenus. Il

est intéressant de noter que les mêmes facteurs qui conduisent à une large barrière intrinsèque, conduisent simultané-

ment à de très grandes forces motrices thermodynamiques pour la fragmentation, ce qui devrait provoquer une diminu-

tion de la barrière d’activation. Cinétiquement, ces effets qui s’opposent l’un à l’autre fournissent une connaissance

plus profonde du processus impliquant les réactions rapides de fragmentation de l’ion radical.

Mots clés : benzothiazoline, cation radical, fragmentation, effets stériques, DFT.

[Traduit par la Rédaction] Shukla et al. 757

Introduction

bond dissociation energy of the radical cation and, in

general, the more likely that fast fragmentation of the corre-

sponding radical cation would be achieved. Conversely, as

the oxidation potential is lowered, the bond dissociation en-

ergy (BDE) of the radical cation increases and will eventu-

ally reach a point where the fragmentation will be too slow

to be competitive with other reactions of the radical cation.

Fragmentations of C—C bonds of several radical cations

One-electron oxidation can decrease bond dissociation en-

ergies of otherwise stable molecules to such an extent that

fast fragmentation can occur readily. This effect is best un-

derstood in terms of a thermodynamic cycle, pioneered by

Arnold and co-workers (1). According to the cycle, the

higher the oxidation potential of the molecule, the lower the

Received 20 March 2003. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 26 June 2003.

Dedicated to Donald R. Arnold, a pioneer in the chemistry of photoinduced electron transfer.

G. Liu and J.P. Dinnocenzo.¹Department of Chemistry, University of Rochester, Rochester, NY 14627-0216, U.S.A.

D. Shukla²and S. Farid.³Research Laboratories, Eastman Kodak Company, Rochester, NY 14650-2109, U.S.A.

¹Corresponding author (e-mail: jpd@chem.rochester.edu).

²Corresponding author (e-mail: deepak.shukla@kodak.com).

³Corresponding author (e-mail: samir.farid@kodak.com).

Can. J. Chem. 81: 744–757 (2003)

doi: 10.1139/V03-078

Shukla et al.

745

have been investigated and it has been confirmed that the

fragmentation rate constants indeed depend strongly on the

oxidation potential of the substrate (2).

[1]

It is conceivable that compounds with low oxidation po-

tentials could yield radical cations that rapidly fragment if

the fragmenting bonds are weakened through proper substi-

tution. There are, in principle, two ways to weaken such

bonds: (1) by having substituents on the fragments (the radi-

cal and the cation) that stabilize these species; and (2) by

steric crowding in the reactants. There are a number of re-

ported examples where fragmentation of radical cations de-

rived from low oxidation potential compounds have been

achieved by such weakening of the fragmenting bonds (3).

Herein, we report a new class of low oxidation potential

compounds — substituted benzothiazolines — that undergo

rapid fragmentation when oxidized to their radical cations.

Pulsed laser techniques were used to generate and character-

ize the benzothiazoline radical cations, as well as to measure

their fragmentation kinetics. In addition, DFT calculations

were performed to estimate the BDEs of the radical cations

as well as their activation barriers for fragmentation. Finally,

the relative contributions to bond weakening in the radical

cations from stabilization of the fragments vs. steric crowd-

ing were determined.

Compound 1 was used as a model nonfragmenting ana-

logue to determine the absorption spectrum of the main

chromphore of the radical cation.

Five other derivatives (2–6) with varying substitution of R

to promote fragmentation, were synthesized to test the effect

of increasing stabilization of the radical R· and of weakening

of the bond on the fragmentation rate constant.

The benzothiazoline derivatives were prepared from 2-

methylamino-benzenethiol according to reported procedures

(eq. [2]) (4, 5) through direct condensation with the appro-

priate ketone (compounds 2 and 3) or through reaction with

an acid chloride to give a thiazolium salt, followed by the

addition of CH₃MgBr (compounds 1 and 4–6).

Results and discussion

Structural features and synthesis

The initial aim of this work was to choose an electrophore

with such a low oxidation potential that cleavage of a non-

stabilized radical fragment upon one-electron oxidation

would be very slow. Systematic structural changes to in-

crease the stability of the radical fragment would then be

tested with the aim of achieving fragmentation of the radical

cation with a rate constant of 1 × 10⁵to 1 × 10⁷s^–1, a conve-

nient range for transient kinetics. Substituted benzothiazo-

line (2,3-dihydrobenzothiazole) derivatives, BT–R (eq. [1])

appeared to be well-suited for this study. They have low oxi-

dation potentials (e.g., 0.77 V vs. SCE for R = Me), the cat-

ion fragment (BT⁺) is highly stabilized, and derivatives with

varying R — the cleaving radical fragment — can be readily

synthesized.

746

Can. J. Chem. Vol. 81, 2003

[2]

Steady state photolysis and reaction products

To correlate the decay of the radical cations to their frag-

mentations, it was important to first confirm that the prod-

ucts of these reactions are those expected according to

eqs. [1] and [3]. The photoreactions of MeOP⁺with the

benzothiazolines 3–6 at equimolar concentrations in CD₃CN

1

were followed by H NMR and the products were identified

by comparison with authentic samples (see Experimental

section for details). The products were indeed those ex-

pected from the N-O fragmentation of MeOP⁺(phenan-

thridine and methanol) and those of the C-C fragmentation

of the radical cations BT–R^·+(2,3-dimethylbenzothiazolium

and products of the cleaved radical R·). In each case the

main product of the radical R· was the dimer R–R (~70 to

>90%). In some cases the deuterated product R–D (via deu-

terium abstraction from the solvent), and (or) the corre-

sponding olefin (via loss of hydrogen atom) were formed

(eq. [4]) (see Experimental section).

Experimental approach

Different approaches were considered to generate and

study the fragmentation kinetics of the radical cations of the

benzothiazoline derivatives (BT–R^·+), by flash photolysis. In

one approach, an excited sensitizer reacts with a donor (act-

ing as a cosensitizer, C) to form the radical cation of the lat-

ter (C^·+) in high quantum yield (6). Next, a secondary

electron transfer from BT–R to C^·+generates BT–R^·+. To

avoid direct excitation of the reactant, it is desirable for the

sensitizer to absorb at longer wavelength than the BT–R de-

rivative. The absorption of BT–R derivatives below 350 nm

renders sensitizers such as 1,4-dicyanonaphthalene and N-

methylquinolinium of limited use. The dynamics of the radi-

cal cations BT–R^·+are most conveniently studied at their ab-

sorption maxima, which, as shown below, are near 420 nm.

Thus, sensitizers such as 9,10-dicyanoanthracene that have

strong absorptions in this region are also of limited use.

Because of the lack of a suitable electron transfer sensi-

tizer with absorption in the 370 ± 10 nm window that would

meet the above requirements, we applied a different ap-

proach to generate the radical cations BT–R^·+. This approach

is based on the chemistry of N-methoxyphenanthridinium

(MeOP⁺), which is reported to undergo photochemical N—O

bond cleavage to yield phenanthridine radical cation (P^·+)

and a methoxy radical (eq. [3]) (7). The radical cation of

phenanthridine is a powerful oxidant (oxidation potential of

phenanthridine is ~1.9 V vs. SCE) that can be used in a sub-

sequent bimolecular reaction to generate the radical cation

of an added donor (eq. [3]).

[4]

An unexpected, minor photolysis product of these reac-

tions was adduct 7 (eq. [4]).⁴This compound was formed in

all reactions of 3–6 in ~12–20% yield. The structure of this

compound, which has not yet been isolated in pure form,

1

was based on H NMR spectral analysis. Importantly, the al-

ternative assignment of the isomeric structure 8 to this com-

pound was ruled out by independent synthesis of 8 via the

base-catalyzed reaction of MeOP⁺with dimethylbenzothia-

zolium (eq. [5]). Assignment of the NMR signals of com-

pound 8 was based on several 2D NMR and NOE

experiments, which establish the connectivity of the two

moieties.

Photolysis of MeOP⁺in the presence of the benzyl deriva-

tive (2) yielded, in addition to phenanthridine and methanol,

2-methyl-3-ethylbenzothiazolium, an expected fragmentation

product of 2^·+. However, none of the likely products of benzyl

[3]

⁴The mechanism for formation of 7 has not been investigated. One possibility involves initial reaction of MeO· with BT–R by hydrogen

atom abstraction from the N-Me group. The resulting α-amino radical might then add to MeOP⁺at C-6 to give a N-alkoxy amine radical

cation. Subsequent intramolecular electron transfer from the benzothiazoline moiety to the N-alkoxy amine radical cation followed by frag-

mentation of the resulting benzothiazoline radical cation would produce 7. Further experiments are required to test this hypothesis.

Shukla et al.

747

Fig. 1. Absorption spectrum of phenanthridine radical cation

(P^·+) obtained from photolysis of MeOP⁺in acetonitrile (closed

circles) and that obtained in the presence of 1,4-dimethoxyben-

zene (DMB) (open circles), which matches the spectrum of an

independently generated DMB^·+.⁵

Fig. 2. Absorption spectra of the radical cations of compounds 1

and 4, obtained from photolysis (380 nm) of MeOP⁺in the pres-

ence of these electron donors in acetonitrile. The spectra were

obtained after a delay time of >2 µs to avoid contributions owing

to the secondary transient species produced from photolysis of

MeOP⁺(see text).

[5]

lysis. Irradiation of MeOP⁺in acetonitrile gave a transient

(λ_maxat 670 nm), which was assigned to P^·+based on the

lack of quenching by dioxygen and on its efficient quench-

ing by 1,4-dimethoxybenzene (DMB, E^ox= 1.3 V vs. SCE).

In the presence of 0.1 M DMB, the 670 nm absorption is

completely replaced by that shown in Fig. 1 (λ_max= 440 and

458 nm), which corresponds to the spectrum of DMB radical

cation generated independently.⁵Using the benzothiazoline

derivatives as donors (eq. [3]) this approach provided a con-

venient method to generate their radical cations and to study

their fragmentation kinetics.

radical, such as 1,2-diphenylethane or toluene were formed

in this reaction. There is also no indication that either benzyl

methyl ether (a coupling product between benzyl and

methoxy radicals) or N-benzylacetamide (an addition prod-

uct of benzyl cation to acetonitrie, followed by hydrolysis) is

formed. In spite of the fact that not all fragmentation prod-

ucts have been identified in this case, the clear evidence for

formation of the benzothiazolium salt suggests that fragmen-

tation of 2^·+does take place. Additional support for this con-

clusion is provided by the fact that the activation enthalpy

calculated for fragmentation of 2^·+agrees well with that

measured experimentally for its disappearance (see Density

functional calculations).

As expected, the phenanthridine radical cation (P^·+) is effi-

ciently intercepted by the benzothiazoline derivatives (k_q≈

6 × 10⁹M^–1s^–1). Thus, 355 or 380 nm laser excitation of

MeOP⁺in the presence of 5–10 mM of 1 in aerated or in ar-

gon-saturated acetonitrile led to a long-lived species (~200–

250 µs) with an absorption maximum at ~410 nm (Fig. 2).

Support for assigning this absorption to 1^·+was obtained

from a number of experiments. Excitation of 1,4-dicyanon-

aphthalene at 343 nm in the presence of 0.3 M biphenyl as a

cosensitizer in acetonitrile led to the formation of biphenyl

radical cation, characterized by the strong absorption at 670

and 380 nm. Absorption due to the sensitizer radical anion

was removed by purging the samples with dioxygen, where

electron transfer leads to the formation of O₂^·–, which does

not absorb in the visible region. Addition of 1 (0.5 to

1.5 mM) led to quenching of the biphenyl radical cation

with a bimolecular rate constant of 1.8 × 10¹⁰M^–1s^–1and

concomitant appearance of an absorption identical to that

Characterization of the radical cations BT–R^·+

The following experiments were carried out to probe the

applicability of MeOP⁺photochemistry (eq. [2]) to generate

and study the reactivity of radical cations by flash photo-

⁵An authentic spectrum of DMB^·+was obtained in aerated acetonitrile by 355 nm excitation of DCA (OD₃₅₅= 0.6) in the presence of 0.15 M

biphenyl (as a cosensitizer) and 0.01 M DMB.

748

Can. J. Chem. Vol. 81, 2003

obtained in the above mentioned experiment with MeOP⁺.⁶

Similarly, 343 nm excitation of N-methylquinolinium with

toluene (1.5 M) as a cosensitizer in dioxygen-purged 1,2-

Fig. 3. (Top) Transient spectra from photolysis (380 nm) of

MeOP⁺in the presence of the methylfluorenyl derivative 5 (0.01

M) in acetonitrile at different delay times after the pulse. The

spectrum after (a) 0.2 µs is essentially that of the radical cation

5^·+, with increasing delay times ((b) and (c)) this spectrum is

gradually replaced by that of methylfluorenyl radical. (Bottom)

Spectrum after (d) 13 µs delay and spectrum of methylfluorenyl

radical obtained from direct photolysis of the dimer (9) (e) at

308 nm in acetonitrile. The spectra from photolysis of MeOP⁺

could not be measured below 400 nm because of absorption by

MeOP⁺.

dichloroethane generated the toluene radical cation (λ_max

=

450 nm), which was efficiently quenched by 1 (~2 mM).

The transient spectrum recorded after complete quenching of

the toluene radical cation was also very similar to that ob-

tained in the other experiments mentioned above.

Assigning the ~410 nm absorbing transient to 1^·+was fur-

ther confirmed through electron transfer interception by a

lower-oxidation-potential compound. The transient obtained

from the reaction of MeOP⁺with 1 in acetonitrile was

readily quenched by tri-p-anisylamine (E^ox= 0.52 V vs.

SCE) with the concomitant formation of the radical cation of

the latter, characterized by its absorption maximum at

720 nm (6). Plots of the rate constants for the transient de-

cays and for growth of the trianisylamine radical cation vs.

amine concentration were linear. Slopes of the plots pro-

vided a quenching rate constant of 4.1 × 10⁹M^–1s^–1.

Shown also in Fig. 2 is the spectrum of 4^·+obtained using

MeOP⁺as described above for 1^·+. Clearly the absorption

spectra of the radical cations are not the same in spite of the

fact that the low-oxidation-potential electrophore (the

benzothiazoline moiety) is the same. The weak absorption

that appears as a shoulder at ~470 nm in the spectrum of the

nonfragmenting 1^·+seems to be considerably enhanced in

the spectrum of 4^·+. The spectra of 2^·+(not shown) and of 5^·+

(Fig. 3) are similar to that of 4^·+.

Additional support for the spectral assignment of the ben-

zothiazole radical cations was derived from the transient as-

signed to 5^·+, whose decay was concomitant with formation

of the 9-methylfluorenyl radical. Unlike the radicals result-

ing from the fragmentation of the other radical cations men-

tioned in this work, which lack absorptions in the visible

region, the methylfluorenyl radical could be readily identi-

fied by its absorption (λ_max= 485 nm) (see Fig. 3) (8). This

absorption matches that of an authentic sample, also shown

in Fig. 3, generated from photolysis of the dimer (compound 9)

(eq. [6]). As expected, in aerated solution this absorption

band is suppressed as a result of quenching by dioxygen.

[6]

Fragmentation rate constants and activation energies

The fragmentation rate constants of the radical cations

BT–R^·+, obtained via 355 or 380 nm excitation of MeOP⁺,

were determined from the decay kinetics in acetonitrile. The

use of MeOP⁺in these flash photolysis experiments allowed

for a high enough concentration (10–15 mM) of the reac-

tants to ensure fast interception of the phenanthridine radical

cation. Under these conditions the growth rates of the radical

cations are faster than their decay. For all samples the

radical cation decays were somewhat complicated by the

presence of a second transient species. The decay rate con-

stant for this latter species was the same in all samples and

had the same temperature dependency. The nature of this

second transient has not been established, but it is likely to

⁶Compound 1 absorbs at the excitation wavelength and concentrations higher than 1.5 mM and could not be used in these experiments.

Shukla et al.

749

Table 1. Comparison of fragmentation rate constants (k_frat

25°C) and activation enthalpies for substituted benzothiazoline

radical cations 2^·+–6^·+and R-H bond dissociation energies.

Fig. 4. Plot of the logarithm of fragmentation rate constants

divided by the absolute temperature (ln (k_fr/T)) for the radical

cations of compounds 3–6 vs. the reciprocal of the absolute tem-

perature (1000/T) in acetonitrile. The negative slopes of the lin-

ear plots obtained by least-squares fitting are: 4.82, 4.84, 2.00,

and 2.15, respectively. The dashed line (negative slope of 2.98)

corresponds to the decay rate constant of another intermediate

from the photolysis of MeOP⁺detected in all cases (see text).

Compound (R-)

k_fr(s^–1)

∆H^‡(BT–R^·+)^a

BDE (R-H)^b

2 (PhCH₂-)

3 (Ph₂CH-)

4 (PhCMe₂-)

5 (9-MeFl-)

6 (Ph₂CMe-)

1

~16^c

89.8

85.8

87.3

79.7

82.8

9.8 × 10⁴

1.9 × 10⁵

5.0 × 10⁶

6.1 × 10⁶

9.5 ± 0.9

9.6 ± 0.5

3.9 ± 0.7

4.4 ± 0.6

Note: All energies in kcal/mol.

^aFrom nonlinear least-squares fit of the temperature-dependent rate data;

reported errors are one standard deviation.

^bReference (9).

^cThis value was estimated from the room temperature fragmentation rate

constant for 2^·+assuming that the activation entropy was the same as for 3^·+.

be due to a species resulting from MeOP⁺chemistry because

of its independence on the added BT–R derivative. Thus, al-

though this experimental approach suffered from the need to

analyze the data as a sum of two exponentials, it offered the

best option to obtain the desired kinetic data.

The fragmentation rate constants of 3^·+–6^·+were in the

range that could be readily determined by laser flash

photolysis. The rate constants of these reactions were also

measured as a function of temperature (see Fig. 4). Listed in

Table 1 are the fragmentation rate constants at 25°C and the

activation enthalpies, which were derived from nonlinear

least-squares fitting of the kinetic data in Fig. 4.

Unlike the fragmentations of 3^·+–6^·+, which could be

readily investigated by flash photolysis, the radical cation 2^·+

had a lifetime of ~50–60 µs and, importantly, its decay

showed only a slight temperature dependence. This behavior

suggests that the decay may be largely due to other deactiva-

tion processes rather than being a measure of the fragmenta-

tion rate constant. Indeed, based on cyclic voltammetry, a

rate constant of only 1 ± 0.2 s^–1was obtained for the frag-

mentation of 2^·+(see Experimental section).

From the above mentioned data it is evident that the frag-

mentation rate constants for 2^·+–6^·+fall into three groups.

Cleavage resulting in formation of the primary, benzyl radi-

cal (from 2^·+) is particularly slow (~1 s^–1). The cleavages to

form both the secondary, diphenylmethyl radical (from 3^·+)

and of the tertiary, cumyl radical (from 4^·+) are in the range

of ~1 × 10⁵s^–1. The cleavages to form the tertiary,

methylfluorenyl, and diphenylethyl radicals (from 5^·+and

6^·+) are in the range of 5 × 10⁶s^–1. The increased substitu-

tion on the radical fragment clearly increases the fragmenta-

tion rate constant.

Although the activation energies for the fragmentation of

2^·+–6^·+roughly correlate with increasing stability of the radi-

cal fragment, further analysis reveals that other factors must

also play a role in controlling the fragmentation barriers.

The relative radical (R·) stabilities can be estimated from the

R—H bond dissociation energies (BDE (R-H)) of the corre-

sponding hydrocarbons, as shown in Table 1. The data show

clear discontinuities in the correlation of BDE (R-H) and

∆H^‡. For example, ∆H^‡for 3^·+and 4^·+are identical within

experimental error yet, as judged by BDE (R-H), the radical

formed from 4^·+is less stabilized than from 3^·+by

1.5 kcal/mol. Similarly, ∆H^‡(5^·+) is 0.5 kcal/mol lower than

∆H^‡(6^·+), however, based on radical stability one would ex-

pect the radical formed from 5^·+to be ca. 3 kcal/mol more

stablized than that formed from 6^·+. These data clearly reveal

that radical stability is not the sole determinant of the rela-

tive energetic barriers for radical cation fragmentation; it

seems likely that differential steric effects also play a crucial

role. To gain insight into the relative contributions of these

two factors, we decided to use density functional theory to

calculate both the bond dissociation energies and the activa-

tion energies for C—C bond fragmentation of 2^·+–6^·+.

Density functional calculations

The thermodynamic and kinetic properties of the substi-

tuted benzothiazoline radical cations were modeled with

density functional calculations. The B3LYP functional was

chosen because it has been shown to be particularly well-

suited to radical cation calculations (10). Computations were

performed with the Gaussian 98 series of programs (11). All

geometries were fully optimized using a 6-31G* basis set

and all optimized species were determined to be either min-

ima or saddle points by frequency calculations. The radical

cation carbon–carbon BDEs for 2^·+–6^·+were determined as

the differences between the energies of the benzothiazolium

cation (BT⁺) and the radical fragments (R·) and that of the

most stable radical cation conformer, unless otherwise noted.

The energy differences include electronic energies, zero

point energy corrections, and thermal corrections (at

298.15 K). In several cases, BDEs from higher energy radi-

cal cation conformers were calculated to compare with the

transition state energies for conformational interconversion.

750

Can. J. Chem. Vol. 81, 2003

the 5^·+/5 pair.⁷The lowest energy conformer for 5^·+is

predicted to be n (see Fig. 5), whereas the lowest energy

conformer of 5 is predicted to be c (E_rel(5_n) = 2.3; E_rel(5_s) =

1.4 kcal/mol). The relative energies of the calculated con-

formers for 5 suggest that over the temperature range used

to measure the fragmentation kinetics of 5^·+(–35 to +21°C),

>90% of 5 should be present as 5_c. Making the reasonable

assumption that one-electron oxidation of 5 does not result

in simultaneous oxidation and conformational intercon-

version, one would expect 5 to be oxidized to initially form

5_c^·+, i.e., not the lowest energy conformer of the radical cat-

ion (5_n^·+). Consequently, if the energy barrier for fragmenta-

Activation energies

The activation energies for fragmentation of 2^·+–6^·+were

sought for comparison with the experimentally determined

values. The relative energies of the conformational isomers

of the radical cations were first computed to determine the

lowest energy conformer for each compound. The energetic

barriers for conformational interconversion were also com-

puted. The results are graphically illustrated in Fig. 5.

The activation energies for fragmentation were computed

from the lowest energy conformer for each radical cation.

For the radical cations with the lowest activation energies for

fragmentation, activation energies were also computed for

the higher energy conformers. The results are shown in

Fig. 5.

·+

tion from 5_cis less than the barrier for conformational

·+

interconversion to the lower energy 5_nconformer, then the

experimental activation barrier for fragmentation of 5^·+

The highest computed barrier for fragmentation

(14.4 kcal/mol) was found for 2^·+. This value is in line with

the observed fragmentation rate constant for 2^·+of ~1 s^–1,

which corresponds to an activation barrier of ~16 kcal/mol,

~2 kcal/mol higher than the computed value. The computed

barriers for 3^·+(7.8 kcal/mol) and 4^·+(7.0 kcal/mol) are sub-

stantially lower. Gratifyingly, the computed barriers for 3^·+

and 4^·+are also consistent with those determined experimen-

tally (9.5 and 9.6 kcal/mol). Again, as in the case of 2^·+, the

measured barriers for 3^·+and 4^·+are higher than the com-

puted values by ca. 2 to 3 kcal/mol.

As shown in Fig. 5, the computed activation energies for

fragmentation of 3^·+and 4^·+begin to become comparable to

the activation energies for conformational interconversion.

This trend continues as one progresses toward the most reac-

tive radical cations (5^·+and 6^·+). In these latter cases, the ac-

tivation energies computed for fragmentations from the

lowest energy conformers are both lower than for conforma-

tional interconversion, particularly for 6^·+. The computed

activation energy for fragmentation of 6^·+(4.7 kcal/mol) is

in excellent agreement with the experimental value

(4.4 kcal/mol) and is ca. 3 kcal/mol less than for 3^·+and 4^·+,

in reasonable agreement with the experimental results (∆∆H^‡≈

5 kcal/mol).

Interestingly, the computed fragmentation barrier from the

lowest energy conformer of 5^·+(6.4 kcal/mol) is intermedi-

ate between those calculated for 4^·+and 6^·+. This contrasts

with the measured activation enthalpy for fragmentation of

5^·+(3.9 kcal/mol), which is more similar to that of 6^·+

(4.4 kcal/mol) than to that of 4^·+(9.6 kcal/mol). A possible

origin of this apparent discrepancy was discovered when

comparing the relative energies of the conformers for 2^·+–6^·+

with those for the corresponding neutral molecules (2–6).

Calculations revealed that the lowest energy conformers for

radical cations 2^·+–6^·+also corresponded to the lowest en-

ergy conformers for the neutral molecules 2–6, except for

should be compared to the computed fragmentation barrier

·+

predicted from 5_cnot from 5_n

.

Figure 5 shows that the computed activation enthalpy for

·+

fragmentation of 5_c(4.4 kcal/mol) is in good agreement

with the experimental value (3.9 kcal/mol). Interestingly, the

computations also show that the energy of the fragmentation

·+

transition state from 5_cis equal to that for conformational

interconversion to 5_n^·+. Thus, the calculational results predict

·+

that a significant fraction of 5_cshould partition to the 5_n

conformer, which is predicted to have a significantly larger

barrier to fragmentation (6.4 kcal/mol). As illustrated in

·+

Fig. 5, if 5_cpartitions between fragmentation and con-

formational interconversion, one would expect the experi-

mental decay of 5^·+to show biexponential behavior, i.e., a

·+

fast component due to fragmentation from 5_cand a slow

component due to fragmentation from 5_n^·+. Unfortunately,

the presence of the unknown species resulting from MeOP⁺

chemistry masked our ability to establish whether the frag-

mentation of 5^·+was biexponential. Finally, it is worth not-

ing that all of the calculated transition state energies refer to

activation enthalpies, whereas the kinetics are obviously de-

termined by the lowest activation free energies. It seems

plausible that the activation entropies for the radical cation

fragmentations might be significantly lower than ∆S^‡for the

conformational interconversions. If so, the activation free en-

·+

ergy for fragmentation of 5_cmight be lower than for

conformational interconversion to 5_n^·+. This also would be

consistent with our kinetic data.

Bond dissociation energies

Gas phase and solution phase bond dissociation energies

were calculated for the benzothiazoline radical cations 2^·+–6^·+.

The gas phase bond dissociation energies (BDE (BT-R^·+)_g),

were determined using B3LYP/6-31G* calculations as the

sum of the enthalpies of the benzothiazolium cation (BT⁺)

⁷The origin of the reversal in conformer stability for 5 vs. 5^·+can be traced to two factors: (i) a change in the geometry at nitrogen upon one-

electron oxidation; and (ii) the conformational inflexibility of the fluorenyl ring. The calculations on neutral molecules 1–6 showed that the

nitrogen atom is strongly pyramidalized. In contrast, the nitrogen is much less pyramidalized in the corresponding radical cations and is ac-

tually planar for 1^·+. The planarization of the nitrogen in radical cations causes the N-CH₃group to move closer to the R groups at C-2, re-

sulting in increased, destabilizing nonbonded interactions. For 5/5^·+, planarization of the nitrogen upon one-electron oxidation resulted in a

severe steric interaction between the N-CH₃group and syn-C-1 hydrogen on the fluorenyl ring for the 5_c^·+conformer, which caused a signif-

icant bow in the ring. In 5 this interaction is greatly diminished by pyramidalization at nitrogen which moves the N-CH₃group away from

the fluorenyl ring. We attribute the interchange in the relative stabilities of the c and n conformers of 5 vs. 5^·+to this structural reorganiza-

tion. Interestingly, in the otherwise structurally similar pair 6/6^·+, a similar steric interaction between the N-CH₃group and one of the hydro-

·+

gens in the syn-phenyl group of 6_cis relieved by rotation of the phenyl group. A comparable structural relaxation for 5_cis not possible

because rotation of the syn-phenyl ring is constrained by its linkage to the other phenyl group that makes up the fluorenyl ring.

Shukla et al.

751

Fig. 5. Results of density functional calculations showing the rel-

ative potential energies (kcal/mol) for the ground state conform-

ers of 2^·+–6^·+and for the transition states that interconnect them.

Also shown are activation enthalpies for fragmentation from se-

lected radical cation conformers (vertical lines). Shown in bold

are the ground and transition states for the proposed, reacting

conformers.

The results are shown in Table 2, along with the

experimental and calculated activation energies for radical

cation fragmentation. The data show that there is a general

correlation of ∆H^‡with the radical cation BDEs.

Table 2 also lists estimates of the solution phase bond dis-

sociation energies for 2^·+–6^·+(BDE (BT-R^·+)_s) which were

obtained by using Arnold’s thermodynamic cycle method

(1), as shown below. For these estimates we have used the

approximation that the oxidations of 2–6 are similar to the

reversible oxidation potential measured of 1 (0.77 V vs. SCE

in CH₃CN). The oxidation of the 2,3-dimethylbenzothiazo-

lium radical was estimated from the (irreversible) reduction

of the corresponding cation (~–1.3 V vs. SCE in CH₃CN).

Finally, the bond dissociation energies of the neutral benzo-

thiazolines 2–6 (BDE (BT-R)_g), were estimated by DFT cal-

culations. The results shown in Table 2 reveal that the

estimated solution phase, radical cation bond dissociation

energies are ca. 7–9 kcal/mol lower than those calculated for

the gas phase. The lower solution phase BDEs would be

consistent with greater solvation of the benzothiazolium cat-

ion than that of the radical cations.

The relative BT–R bond dissociation energies for 2^·+–6^·+

presumably reflect contributions from differences in both the

stabilization of the radicals and the steric crowding present

in the radical cations. We sought to estimate the comparative

contributions of these factors to the relative BDEs. Relative

radical stabilization energies of R· can be defined in a num-

ber of ways. One of the most general definitions has been pi-

oneered by Rüchardt, who has compared the difference in

stability between a pure hydrocarbon radical and its substi-

tuted analogue in which one or more alkyl groups are re-

placed by substituents (12). The advantage of this approach

is that it provides radical stabilization increments for sub-

stituents that are transferable between different radicals be-

cause the intrinsic differences in stabilities for primary,

secondary, and tertiary radicals are factored out. To assess

the relative effects of radical stabilities on the BDEs for 2^·+–

6^·+, however, we do want to include the differences in stabil-

ities between primary, secondary, and tertiary radicals.

Therefore, we have adopted the more traditional approach to

assessing radical stabilities by comparing the relative R-H

BDEs to a reference compound. We are primarily interested

in only the relative contributions to radical stability along

the series, therefore, we have chosen the least stabilized rad-

ical in this series (PhCH₂·) as our reference. Thus, we define

and the radical fragments (R·) minus the enthalpy of the

most stable radical cation conformer. For 5^·+, the enthalpy of

the 5_c^·+conformer was used for the reasons described above.

752

Can. J. Chem. Vol. 81, 2003

Table 2. Experimental (∆H^‡_exp) and calculated (∆H^‡_calcd) activation energies for fragmentation of 2^·+–6^·+, gas phase C-R bond dissocia-

tion energies calculated for 2–6 (BDE (BT-R)_g), gas phase (BDE (BT-R^·+)_s), and solution phase (BDE (BT-R^·+)_s) bond dissociation en-

ergies calculated for 2^·+–6^·+.

a

Compound (R-)

∆H^‡

BDE (BT-R)_g

BDE (BT-R^·+)_g

BDE (BT-R^·+)_s^b

exp

calcd

11.6

–0.6

–0.7

–8.5

–10.3

~5

~–10

~–9

~–16

~–19

2 (PhCH₂-)

3 (Ph₂CH-)

4 (PhCMe₂-)

5 (9-MeFl-)

6 (Ph₂CMe-)

~16

9.6

3.9

4.3

14.4

7.8

7.0

4.4

4.7

52.2

37.8

38.4

32.1

28.3

Note: All energies in kcal/mol.

^aFrom density functional calculations.

^bBDE (BT-R)_g– E^ox(BT-R) + E^ox(BT^·); based on the approximation that E^ox(BT-R) is the same for 2–6.

Table 3. Estimates of the contributions of radical stability and steric effects on the gas phase and solution BT-R bond dissociation en-

ergies for 3^·+–6^·+relative to those for 2^·+.

Relative radical

stabilization^a

Relative steric

Radical Cation

R-

effect (gas phase)^b

effect (solution)^c

2^·+

3^·+

4^·+

5^·+

6^·+

PhCH₂-

Ph₂CH-

PhCMe₂-

9-MeFl-

Ph₂CMe-

(0)

–8

–10

–15

(0)

–4

–3

–10

–7

–11

–17

Note: All energies in kcal/mol.

^aBDE (PhCH₂-H) – BDE (R-H); see Table 1 for values.

^b{[BDE (BT-CH₂Ph^·+)_g– BDE (BT-R^·+)_g] – [BDE (PhCH₂-H) – BDE (R-H)]}.

^c{[BDE (BT-CH₂Ph^·+)_s– BDE (BT-R^·+)_s] – [BDE (PhCH₂-H) – BDE (R-H)]}.

relative radical stabilities for the present purpose as BDE

(PhCH₂-H) – BDE (R-H).⁸For the relative steric contribu-

tions to the BDEs for 2^·+–6^·+, we also use the benzyl substi-

tuted radical cation as a reference. Thus, the relative steric

effects are estimated by subtracting the radical stabilization

contribution from the difference in radical cation BDEs, i.e.,

{[BDE (BT-CH₂Ph^·+) – BDE (BT-R^·+)] – [BDE (PhCH₂-H) –

BDE (R-H)]}. The results of this analysis are shown in Ta-

ble 3. Note that the relative steric effects estimated from the

gas phase and solution BDEs for 2^·+–6^·+are in good agree-

ment.

trinsic barriers for fragmentation (≤4 kcal/mol), showing that

the internal and solvent reorganization energies for the reac-

tions are quite small. In this respect the fragmentations of

3^·+–6^·+are clearly different. Based on the fact that 3^·+and 4^·+

have substantial activation energies for fragmentation

(∆H^‡≈ 10 kcal/mol) despite the reactions being exothermic

by 9 to 10 kcal/mol in solution, one can conclude that the in-

trinsic barriers for fragmentation of the substituted benzo-

thiazline radicals studied here are >10 kcal/mol.

The results in Table 3 show that, relative to 2^·+, the BDEs

for 3^·+–6^·+roughly fall into two classes. For 5^·+, the relative

contributions owing to radical stabilization and steric effects

are nearly equal, 10 and 11 kcal/mol, respectively. For radi-

cal cations 3^·+, 4^·+, and 6^·+, the relative radical stabilization

and steric contributions to bond weakening are 3–7 kcal/mol

and 11–17 kcal/mol, respectively, i.e., the steric effects dom-

inate.

Origin of the radical cation fragmentation barriers

Estimates of the BDEs for radical cations 3^·+–6^·+in Ta-

ble 2 show that all are predicted to be exothermic in solu-

tion. Nonetheless, the radical cations have substantial

activation barriers to fragmentation. It is of interest to com-

pare this behavior to that observed for the fragmentations of

substituted bibenzyl radical cations (10^·+) reported by

Maslak et al. (3a) and to the alkylated NADH radical cations

(11^·+) reported by Savéant and co-workers (13). These radi-

cal cation fragmentations were found to have very small in-

What is the origin of the difference in the intrinsic barriers

for fragmentation of 10^·+and 11^·+vs. 2^·+–6^·+? We propose

that the difference is related to the relative internal reorgani-

zation energies of the fragmentations. It is perhaps easier to

think about the problem by considering the origin of the bar-

rier for the reverse reaction, i.e., cation + radical recombina-

tion. The barrier for the recombination reaction will be

directly related to the extent of delocalization of the cation

and radical fragments. The greater the degree of delocali-

⁸Although it would have been desirable to use BDE (R-CH₃) rather than BDE (R-H) to estimate relative radical stabilities, the former values

are not well determined for the systems studied here.

Shukla et al.

753

zation, the greater will be the degree of electronic, internal

reorganization, and, therefore, the greater will be the reac-

tion barrier. When comparing the cation and radical frag-

ments produced from fragmentation of 10^·+vs. 2^·+–6^·+, the

primary difference is likely related to the extent of delocali-

zation of the cation, i.e., the benzyl cation (in the case of

10^·+) vs. the benzothiazolium cation (in the case of 2^·+–6^·+).

The benzothiazolium cation is expected to have significantly

less carbocation character at the recombination site than a

benzyl cation, which would lead to a larger internal reorga-

nization energy for the benzothiazolium cation + radical re-

combination. In addition, for benzothiazoline radical cations

3^·+–6^·+, the radicals produced by fragmentation are more

delocalized than a simple benzyl radical, which will also

lead to greater internal reorganization energies for recombi-

nation and, consequently, for fragmentation too. For the

fragmentation of 11^·+vs. 2^·+–6^·+, we presume that delocali-

zation of the cation fragments for the two systems is some-

what similar. However, the relative extent of radical

delocalization is clearly different. For 11^·+, the radical frag-

ment — ·t-Bu — is clearly much more localized than any of

the radicals formed from fragmentation of 2^·+–6^·+. Thus, cat-

ion + radical recombination to form 11^·+should have a

lower internal reorganization energy than the recombinations

to form 2^·+–6^·+.

These factors lead to opposing kinetic effects. On the one

hand, the natural exothermicities of the reactions should de-

crease the activation barriers for fragmentation according to

the Bell–Evans–Polanyi principle. On the other hand, the

large intrinsic barriers increase the activation energies. This

analysis reveals a fundamental limitation of using deloca-

lized cation and radical products in the design of fast radical

cation fragmentation reactions. As shown in the present

work, steric effects are the more attractive structural feature

to utilize for designing fast radical ion fragmentations be-

cause they do not increase the intrinsic barrier. Third, for

relatively fast radical ion fragmentations of sterically con-

gested systems such as those described in this work, one

should keep in mind that the energetic barriers for fragmen-

tation can be lower than the barriers for conformational

interconversion. In these cases, the possibility exists that the

most stable radical ion conformer may not necessarily be the

reactive conformer.

Experimental section

General method

¹H NMR spectra were recorded with either a General

Electric/Nicolet QE-300 spectrometer or a Brüker Avance-

400 spectrometer. ¹³C NMR spectra were recorded with an

Avance-400 spectrometer. Proton chemical shifts (δ) are re-

ported in parts per million (ppm) downfield from tetra-

methylsilane or in ppm relative to the singlet at 7.24 ppm for

the residual CHCl₃in the chloroform-d or the multiplet at

1.93 ppm for the residual CHD₂CN in the acetonitrile-d₃.

Reported proton–proton coupling constants assume first-

order behavior. Splitting patterns are designated as singlet

(s), doublet (d), triplet (t), quartet (q), multiplet (m), and

broad (br). The meta- and para-couplings are ignored.

Ortho-, meta-, and para-hydrogens of phenyl groups are des-

ignated o-, m-, and p-; for magnetically nonequivalent

phenyl groups the signals of the second phenyl group are

designated o′-, m′-, and p′-. The aromatic hydrogens of the

benzothiazoline and benzothiazolium derivatives are num-

bered 4–7 from the α position to N. Carbon chemical shifts

are reported in ppm relative to internal acetonitrile-d₃

(117.61 and 0.60 ppm) or chloroform-d (77.34 (t) ppm).

Acetonitrile was refluxed over CaH₂for 24 h and distilled

fresh prior to use under an atmosphere of nitrogen. Anhy-

drous diethyl ether and tetrahydrofuran were freshly distilled

from benzophenone ketyl under nitrogen.

The proposition that the cation + radical recombination

reactions to form 2^·+–6^·+should have significant barriers is

supported by the fact that even addition of a localized, pri-

mary radical to benzothiazolium cation 12 has a rate con-

stant of only 1 × 10⁶M^–1s^–1at 25°C (14). It should be clear

that addition of the delocalized radicals, such as those de-

rived from fragmentation of 2^·+–6^·+, would have significantly

lower rate constants. Consequently, the fragmentations of

2^·+–6^·+are expected to have significant intrinsic barriers.

Conclusions

There are three main conclusions from this work. First,

we have demonstrated that benzothiazoline derivatives,

which are among the lowest oxidation potential compounds

yet investigated as candidates for radical cation fragmenta-

tion, can undergo rapid fragmentation upon one-electron ox-

idation if the fragmenting bond is sufficiently weakened by a

combination of stabilizing the radical fragment and by steric

crowding in the reactants. These two factors allow the rate

constants for fragmentation of benzothiazoline radical cat-

ions to be tuned by over 6 orders of magnitude. Second,

benzothiazoline radical cations were found to have consider-

ably large intrinsic activation barriers (>10 kcal/mol) for

fragmentation. The barriers were attributed to large reorgani-

zation energies of the product fragments, the benzothiazo-

lium cation and the resonance-delocalized radicals.

Consequently, the same factors that lead to the thermody-

namic driving forces for the fragmentations — the stabilities

of the benzothiazolium cation and the resonance-delocalized

radicals — simultaneously result in large intrinsic barriers.

Electrochemical measurements

Cyclic voltammetric measurements were carried out on a

glassy carbon disk electrode using a CHI660 electrochemi-

cal analyzer (CH Instruments, Inc.) in acetonitrile with

tetrabutylammonium perchlorate as electrolyte. 2,3-Di-

methylbenzothiazolium showed irreversible reduction with a

peak potential of –1.30 V vs. SCE. A reversible oxidation

potential was obtained for compound 1 (0.77 V vs. SCE,

3 mm electrode at 0.5 V/s). Compounds 3 and 6 showed ir-

reversible oxidation with peak potentials at 0.63 and 0.65 V

vs. SCE, respectively.

Using a 33 µm carbon electrode at a scan rate ≥25 V/s,

compound 2 showed a quasi-reversible oxidation of 0.70 V

vs. SCE. Scanning at slower rates (2.5–10 V/s) and with

754

Can. J. Chem. Vol. 81, 2003

varying delays, a rate constant of 1 ± 0.2 s^–1was obtained

for the follow-up chemical reaction, presumably the frag-

mentation of 2^·+.

acetone, cumene, α-methylstyrene, 1,1-diphenylethylene,

and fluorene were all obtained from Aldrich Chemical Com-

pany and used without further purification. 1,1,2,2-Tetra-

phenylethane (16), 2,2,3,3-tetraphenylbutane (17), 2,3-

dimethyl-2,3-diphenylbutane (18), and compound 9 (19)

were prepared following previously reported methods. 2-

Phenyl-2-methylpropionoyl chloride was prepared from the

corresponding acid (20a) by reaction with PCl₅using a pro-

cedure analogous to that described in ref. (20b). Products of

Laser flash photolysis

Nanosecond laser flash photolysis experiments were car-

ried out using either 380 or 355 nm excitations. For 380 nm

excitation, a Lambda Physik Lextra 50 excimer laser (XeCL,

308 nm; <180 mJ/pulse; 10 ns pulses) and a Lambda Physik

LPD 3002 pumped-dye laser (Exciton-384 dye for 380 nm;

<10 mJ/pulse; 7 ns pulses) were used. For 355 nm excita-

tion, the frequency-tripled output from an OPOTEK Vibrant

Q-switched Nd:YAG laser system (pulse width 4 ns, 2–8 mJ)

was used. The excitation pulses were attenuated, when nec-

essary, using neutral density filters.

Transient absorption was monitored at right angles to the

excitation. A pulsed Oriel 150 W xenon lamp (Model

66007) was used as the monitoring beam. The analyzing

beam was collected and focused on the entrance slit (2 nm)

of an Instrument S. A. H-20 monochromator. A Hamamatsu

R-446 photomultiplier tube (PMT) in a custom housing

(Products for Research) was attached to the exit slit of the

monochromator. A computer-controlled Stanford Research

Systems high voltage power supply (model PS310) was used

with the PMT. The signals from the PMT were digitized us-

ing a Tektronix TDS 620 oscilloscope and transferred to a

PC, via a GPIB interface, for data storage and processing. A

Quantum Composer pulse generator (Model 9318) provided

TTL trigger pulses to control the timing for the laser, lamp,

and oscilloscope.

1

the cleaved radicals were identified by comparison with H

NMR spectra of authentic samples.

2,3-Dimethylbenzothiazolium hexafluorophosphate

This material was prepared by modification of a literature

procedure (4). To a stirred solution of acetyl chloride (5.0 g,

63.7 mmol) in benzene (dry, 200 mL), a solution of 2-

methylamino-benzenethiol (8.84 g, 63.7 mmol) in benzene

(20 mL) was added dropwise at room temperature. The reac-

tion mixture was refluxed for 1 h. Solvent removal provided

a pale yellow solid that was dissolved in water (200 mL) fol-

lowed by addition of potassium hexafluorophosphate (7.0 g

in 100 mL H₂O) to afford a white precipitate (19 g, 79%).

¹H NMR (CD₃CN) δ: 8.24 (d, J = 8 Hz, 1H), 8.05 (d, J =

9 Hz, 1H), 7.92 (t, J = 8 Hz, 1H), 7.82 (t, J = 8 Hz, 1H),

4.15 (s, 3H), 3.09 (s, 3H).

N-Methoxyphenanthridinum hexafluorophosphate

N-Methoxyphenanthridinum tetrafluorboate was prepared

from the reaction of phenanthridine-N-oxide with trimethyl-

oxonium tetrafluorborate in dichloromethane (7). A solution

of the BF₄salt (1 g) in ~15 mL water was added to an aque-

ous solution of potassium hexafluorophosphate (2 g in

~10 mL). The mixture was cooled and the precipitate was

recrystallized from water as a crystalline pale yellow solid.

¹H NMR (CD₃CN) δ: 10.09 (s, 1H), 9.06 (d, 1H), 9.02 (d, 1H),

8.57 (t, 2H), 8.43 (t, 1H), 8.24–8.13 (m, 3H), 4.61 (s, 3H).

Appropriate long pass filters were placed on either side of

the sample to prevent photolysis by the analyzing light.

Spectral measurements

To acetonitrile solutions of N-methoxyphenanthridinium

hexafluorphosphate (MeOP⁺) with an OD of 0.5–0.77 at the

excitation wavelength (380 or 355 nm) were added benzo-

thiazolines 2–6 (5–15 mM). The samples were thoroughly

purged with either dioxygen or argon (~10 min) before pass-

ing them through a 1 × 1 cm²quartz flow cell. The flow rate

was adjusted so that fresh sample was available for each la-

ser shot.

Kinetics measurements

The samples were purged with either dioxygen or nitrogen

prior to, and during, laser irradiation. Typically, a solution of

MeOP⁺in acetonitrile (OD at 380 nm = 0.6–0.8) was ex-

posed to 380 nm laser excitation (0.5–0.6 mJ/pulse) in the

presence of the benzothiazoline derivative (6 to 7 mM), and

the decay of the radical cation was followed between 420–

450 nm on the ns to µs time scale. For temperature depend-

ence studies, fresh samples were equilibrated at each tem-

perature for 15–20 min prior to the measurement. Each set

of experiments was repeated at least three times.

2,2,3-Trimethylbenzothiazoline (1)

This compound was prepared according to a literature

procedure (5). To a stirred suspension of 2,3-dimethyl-ben-

zothiazolium toluene-4-sulfonate (5.0 g, 14.32 mmol) in an-

hydrous diethyl ether (125 mL) at –40°C, methylmagnesium

bromide (2.87 M solution in diethyl ether, 5.3 mL, 15 mmol)

was added dropwise over 5 min under an atmosphere of N₂.

The reaction mixture was refluxed for 2 h and then

quenched with aqueous ammonium chloride solution

(70 mL). The organic layer was separated and the aqueous

layer was extracted with ether (3 × 50 mL). The organic ex-

tracts were combined, washed successively with water (2 ×

100 mL) and with brine solution (2 × 100 mL), and dried

Materials

2-Methylamino-benzenethiol was prepared according to a

literature procedure by reductive ring opening of 2,3-

dihydro-1,3-benzothiazole (Aldrich) using lithium aluminum

hydride (15). Methyl iodide, trimethyloxonium tetrafluoro-

borate, acetophenone, 1-phenyl-2-butanone, 1,1-diphenyl-

Shukla et al.

755

over anhydrous magnesium sulfate. Filtration and removal

of the solvent yielded a red oil (2.2 g, 77%). The crude

product was distilled under reduced pressure, bp 80°C at

10 mmHg (1 mmHg = 133.322 Pa) (lit. (5c) bp 75–80°C at

10 mmHg (1 mmHg = 133.322 Pa)) to yield a colorless oil

(2.0 g, 76%). MS m/z: 179 (M⁺, calculated for C₁₁H₁₅NS).

¹H NMR (CDCl₃) δ: 7.05 (d, J = 7.4 Hz, H-7), 6.99 (t, J =

7.6 and 7.8 Hz, H-5), 6.64 (t, J = 7.4 and 7.6 Hz), 6.31 (d,

J = 7.8 Hz, H-4), 2.72 (s, NCH₃), 1.65 (s, C(CH₃)₂). ¹³C

NMR (CDCl₃) δ: 146.64, 124.17, 121.70, 117.55, 106.31,

61.03, 37.28, 28.58, 28.40.

diethyl ether (dry, 850 mL) to form a yellow precipitate. The

precipitate was dissolved in water (400 mL) at 70–80°C and

an aqueous solution of potassium hexafluorophosphate

(4.8 g in 100 mL H₂O) was added. Upon cooling the solu-

tion, a pale yellow precipitate formed. Recrystallization

from acetonitrile – diethyl ether (1:10) gave a colorless solid

of the 2-cumyl-3-methyl-benzothiazolium salt (9.0 g, 83%).

¹H NMR (CD₃CN) δ: 8.35 (d, J = 8.0 Hz, H-7), 8.02 (d, J =

8.3 Hz, H-4), 7.94 (t, J = 7.3 and 8.3 Hz, H-5), 7.88 (t, J =

7.3 and 8.0 Hz, H-6), 7.5–7.42 (m, m- and p-H, 3H), 7.37–

7.35 (m, o-H, 2H), 3.75 (s, NCH₃), 2.04 (s, C(CH₃)₂). ¹³C

NMR (CD₃CN) δ: 142.10, 130.10, 129.62, 128.25, 125.82,

123.79, 117.30, 116.75, 45.72, 37.92, 28.73. ¹³C DEPT45

NMR (CD₃CN) δ: 130.10, 129.62, 128.91, 128.25, 125.82,

123.79, 116.75, 37.92, 28.73.

2-Benzyl-2-ethyl-3-methyl-benzothiazoline (2)

A

solution of 2-methylamino-benzenethiol (1.0 g,

7.2 mmol) and 1-phenyl-2-butanone in absolute ethanol (7 mL)

was refluxed for 48 h under an atmosphere of N₂. The reac-

tion mixture was cooled and kept at –40°C for 3 h. A precip-

itate came out of solution, which rapidly melted at room

temperature to afford a yellow oil. The latter was distilled

under reduced pressure (165–167°C at 0.04 mmHg

(1mmHg = 133.322 Pa)) to obtain a very pale yellow oil

(1.2 g, 63%). MS m/z: 269 (M⁺, calculated for C₁₇H₁₉NS).

¹H NMR (CDCl₃) δ: 7.36–7.26 (m, 5H), 7.04 (d, J = 7.4 Hz,

1H), 7.00 (t, J = 7.6 and 7.8 Hz, 1H), 6.64 (t, J = 7.4 and

7.6 Hz, 1H), 6.30 (d, J = 7.8 Hz, 1 H), 3.28 (d, J = 13.4 Hz,

1H), 3.10 (d, J = 13.4 Hz, 1H), 2.86 (s, 3H), 2.10 (sextet,

J = 14.9 and 7.2 Hz, 1H), 1.72 (sextet, J = 14.9 and 7.2 Hz,

1H), 1.05 (t, J = 7.2 Hz, 3H). ¹³C NMR (CDCl₃) δ: 148.17,

136.36, 131.10, 127.74, 126.62, 125.17, 124.17, 121.09, 117.60,

105.48, 85.78, 45.46, 30.19, 29.04, 9.15. ¹³C DEPT135

NMR (CDCl₃) δ: 131.10, 127.74, 125.17, 121.09, 117.60,

105.48, 45.46 (negative peak), 30.19 (negative peak), 29.04,

9.15.

To a stirred suspension of 2-cumyl-3-methyl-benzothiazo-

lium hexafluorophosphate (1.0 g, 2.42 mmol) in tetrahydro-

furan (80 mL) under an atmosphere of N₂, methylmagnesium

bromide (2.73 M solution in ether, 7.5 mL, 22 mmol) was

added dropwise over 15 min at –70°C. The reaction was

stirred at –70°C for an additional 30 min, followed by 4 h at

room temperature, and then refluxed for 15 min. The reac-

tion was quenched with aqueous ammonium chloride solu-

tion (50 mL, pH ~ 4) and extracted with diethyl ether (3 ×

50 mL). The ether extracts were washed with brine solution

(2 × 50 mL), dried over anhydrous magnesium sulfate, fil-

tered, and the solvent removed to afford a pale yellow oil

(0.54 g, ~79%). The crude material was recrystallized from

aqueous methanol to yield a pale yellow solid, mp 70–71°C.

MS m/z: 283 (M⁺, calculated for C₁₈H₂₁NS). ¹H NMR

(CDCl₃) δ: 7.56 (d, o-H, 2H), 7.34 (t, m-H, 2H), 7.27 (t, p-

H, 1H), 6.99–6.95 (m, 2H), 6.62 (t, 1H), 6.31 (d, 1H), 2.54

(s, 3H), 1.78 (s, 3H), 1.62 (s, 3H), 1.54 (s, 3H). ¹³C (CDCl₃)

δ: 149.95, 144.51, 128.81, 127.23, 126.31, 125.66, 124.90,

119.81, 117.74, 107.0, 87.03, 49.99, 34.06, 24.76, 24.64,

23.49. ¹³C DEPT135 (CDCl₃) (only CH and CH₃carbons

observed) δ: 128.81, 127.23, 126.31, 124.90, 119.81, 117.74,

106.99, 34.07, 24.76, 24.64, 23.49.

2,3-Dimethyl-2-diphenylmethyl-benzothiazoline (3)

A

solution of 2-methylamino-benzenethiol (2.0 g,

14.38 mmol) and 1,1-diphenylacetone (3.02 g, 14.38 mmol)

in absolute ethanol (15 mL) was refluxed for 52 h under an

atmosphere of N₂. Upon cooling to –40°C, a colorless solid

(2.3 g, 48%) formed which was recrystallized twice from

ethanol to yield colorless plates (2.0 g, 42%), mp 82°C. MS

m/z: 331.0 (M⁺, calculated for C₂₂H₂₁NS). ¹H NMR (CDCl₃)

δ: 7.67 (o-H, 2H), 7.53 (o′-H, 2H), 7.33 (m′-H, 2H), 7.27

(p′-H, 1H), 7.20 (m-H, 2H), 7.14 (p-H, 1H), 6.99 (d, J =

7.4 Hz, 1H), 6.92 (t, J = 7.5 and 7.8 Hz, 1H), 6.59 (t, J = 7.4

and 7.5 Hz, 1H), 6.14 (d, J = 7.8 Hz, 1H), 4.51 (s, CHPh₂),

2.69 (s, N-CH₃), 1.75 (s, CH₃). ¹³C NMR (CDCl₃) δ: 147.71,

140.69, 129.89, 128.23, 127.70, 127.03, 126.31, 125.25,

124.06, 121.00, 117.51, 105.99, 83.77, 59.53, 29.14, 27.81.

¹³C DEPT45 NMR (CDCl₃) (only CH and CH₃carbons) δ:

129.89, 129.83, 128.23, 127.70, 127.03, 126.31, 125.25,

121.00, 117.51, 105.99, 59.53, 29.14, 27.81.

2-(1,1-Diphenylethyl)-2,3-dimethyl-benzothiazoline (5)

Reaction of 2,2-diphenylpropionyl chloride with 2-

methylamino-benzenethiol, as described above for com-

pound 4 (reflux time of 5 h), gave the corresponding

benzothiazolium chloride in 70% yield as colorless cubes

1

from acetonitrile – diethyl ether (1:10). H NMR (CDCl₃) δ:

8.57 (d, 1H), 8.12 (d, 1H), 7.87 (t, 1H), 7.72 (t, 1H), 7.48–

7.44 (m, 6H), 7.25–7.21 (m, 4H), 4.29 (s, 3H), 2.66 (s, 3H).

¹³C NMR (CDCl₃) δ: 186.79, 144.05, 143.36, 141.55,

130.76, 129.17, 127.79, 123.25, 118.27, 53.64, 40.54, 27.68.

Similarly, compound 5 was prepared from the reaction of

2-(1,1-diphenylethyl)-3-methylbenzothiazolium chloride with

MeMgBr as described above for compound 4. Compound 5

was obtained in 46% yield as colorless crystals from etha-

nol, mp 58–62°C. MS m/z: 345 (M⁺, calculated for

2-Cumyl-2,3-dimethyl-benzothiazoline (4)

To a stirred solution of 2-phenyl-2-methylpropionoyl

chloride (4.8 g, 26.3 mmol) in benzene (dry, 80 mL), a solu-

tion of 2-methylamino-benzenethiol (3.7 g, 26.4 mmol) in

benzene (20 mL) was added dropwise at room temperature.

The reaction mixture was refluxed for 1.5 h. Solvent re-

moval gave an oil that was dissolved in CH₂Cl₂(100 mL).

The resulting solution was added dropwise to rapidly stirred

1

C₂₃H₂₃NS). H NMR (CDCl₃) δ: 7.37–7.17 (m, 10H), 6.98

(d, J = 7.4 Hz, 1H), 6.97 (t, J = 7.5 and 7.8 Hz, 1H), 6.63 (t,

J = 7.4 and 7.5 Hz, 1H), 6.31 (d, J = 7.8 Hz, 1H), 2.20 (br. s,

3H), 2.06 (s, 3H), 1.98 (s, 3H). ¹³C NMR (CDCl₃) δ: 153.83,

149.34, 133.55, 130.81, 129.35, 127.39, 127.09, 126.51,

125.68, 125.05, 119.85, 107.62, 34.32, 27.58, 25.65.

756

Can. J. Chem. Vol. 81, 2003

2,3-Dimethyl-2-(9-methylfluoren-9-yl)-benzothiazoline (6)

9-Methylfluorene-9-carbonyl chloride (prepared in an

analogous manner to 2-phenyl-2-methylpropionoyl chloride,

see above) was reacted with 2-methylamino-benzenethiol, as

described above under compound 4 (1 h reflux time), to give

the corresponding benzothiazolium hexafluorophosphate salt

Photolysis products of 6

2,3-Dimethybenzothiazolium (88%), phenanthridine (65%),

2,2,3,3-tetraphenylbutane (82%), 1,1-diphenylethylene (12%),

adduct 7 (22%), CH₃OH(D) (63%).

Attempted isolation of adduct 7

1

An equimolar (0.01 M) solution of benzothiazoline 3 and

MeOP⁺in acetonitrile (25 mL) was irradiated under nitrogen

for 20 min in a Rayonet reactor equipped with 400 nm

lamps. The solution was concentrated to ~2 mL and diluted

with diethyl ether (~50 mL). A precipitate formed which

as colorless crystals in 96% yield. H NMR (CDCl₃) δ: 8.89

(d, J ≈ 8 Hz, 1H), 8.30 (d, J ≈ 8 Hz, 1H), 7.91 (d, J ≈

7.6 Hz, 2H), 7.79 (t, J ≈ 7 and 8 Hz, 1H), 7.71 (t, J ≈ 7 and

8 Hz, 1H), 7.54 (t, 2H), 7.41–7.36 (m, 4H), 3.30 (s, 3H),

2.22 (s, 3H). ¹³C NMR (CDCl₃) δ: 181.55, 144.38, 143.32,

139.78, 130.70, 130.07, 129.51, 128.87, 128.73, 125.57,

123.88, 121.70, 117.01, 54.21, 36.37, 29.06.

1

was collected and identified by H NMR as 2,3-dimethyl-

benzothiazolium. Upon addition of hexane to the ethereal

solution the mixture turned turbid and, upon standing, re-

sulted in formation of an oil. The supernatant liquid was de-

canted and the oil was repeatedly washed with hot hexane to

further remove phenanthridine and 1,1,2,2,-tetraphenyl-

The benzothiazolium salt was treated as described above

for compound 4 to give compound 6 as a colorless viscous

oil in 40% yield after flash chromatography (silica gel, 60%

hexane – diethyl ether). MS m/z: 343 (M⁺, calculated for

1

ethane. Analysis of the hexane-washed solid by H NMR

C₂₃H₂₁NS). H NMR (CDCl₃) δ: 7.89 (d, J = 7.6 Hz, 1H),

showed that it was a mixture of primarily 2,3-dimethybenzo-

thiazolium salt and adduct 7, along with traces of 1,1,2,2,-

tetraphenylethane and phenanthridine.

7.78 (d, J = 7.7 Hz, 1H), 7.74 (d, J ≈ 7.5 Hz, 1H), 7.72 (d,

J ≈ 7.5, 1H), 7.41 (t, J = 7.5 Hz, 1H), 7.36 (t, J = 7.5 Hz,

1H), 7.30 (t, J = 7.4 Hz, 1H), 7.22 (t, J = 7.5 Hz, 1H), 7.01

(d, J = 7.4 Hz, H-4), 6.95 (t, J = 7.4 and 7.9 Hz, H-6), 6.66

(t, J = 7.4, 7.4 Hz, H-5), 6.32 (d, J = 7.9 Hz, H-7), 2.61 (s,

3H), 1.79 (s, 3H), 1.62 (s, 3H). ¹³C NMR (CDCl₃) δ: 150.21,

148.91, 148.58, 141.215, 140.97, 127.68, 127.52, 126.85,

126.77, 126.45, 126.01, 125.02, 125.98, 119.84, 119.61,

119.17, 118.49, 108.36, 85.96, 61.87, 34.58, 23.35, 20.75.

Compound 7

¹H NMR (CD₃CN) δ: 8.18 (d, J = 6.3 Hz, 1H), 7.85–7.77

(m, 5H), 7.48 (t, J = 7.7 Hz, 1H), 7.37 (t, J = 7.7 Hz, 1H),

7.24 (d, J ≈ 8 Hz, 1H), 7.21 (t, J = 7.5 Hz, 1H), 7.10 (t, J =

7.4 Hz, 1H), 7.89 (br d, 6.4, 1H), 5.30 (dd, J = 4.8 and

7.4 Hz, 1H), 4.96 (dd, J = 4.8 and 14.6, 1H), 4.75 (dd, J =

7.4 and 14.6 Hz, 1H), 3.80 (s, 3H), 2.53 (s, 3H).

General procedure for photolysis of benzothiazolines in

acetontrile-d₃

Preparation of adduct 8

An equimolar (~0.01 M) solution of the benzothiazoline

and MeOP⁺in acetonitrile-d₃was purged with nitrogen for

5–10 min and then irradiated for 5–10 min in a Rayonet re-

actor equipped with 400 nm lamps. The color of the reaction

mixture changed from pale yellow to slightly darker yellow

during the photolysis. After photolysis, a known amount of

internal standard, viz., tetrachloroethane was added into the

reaction mixture and ¹H NMR recorded. Products were iden-

tified by comparison with ¹H NMR spectra of authentic sam-

ples prepared independently. The yields of products were

To an equimolar solution of N-methoxyphenanthridinium

hexafluorophosphate (35.5 mg, 0.1 mmol) and 2,3-dimethyl-

benzothiazolium hexafluorphosphate (30.9 mg, 0.1 mmol) in

4 mL CD₃CN, ~30 mg anhydrous sodium carbonate was

added. The solution was stirred at room temperature and the

1

reaction was monitored by H NMR. After ~8 h, the conver-

sion was nearly complete yielding almost exclusively com-

pound 8, which was identified by its NMR spectrum

(CD₃CN). The chemical shifts of the ¹H NMR were assigned

based on NOE, COSY, and TOCSY experiments using a

500 MHz spectrometer; those of strongly overlapping sig-

nals were further confirmed by comparison with simulated

spectra. The NOE experiment indicated the proximity of H-6

(methine triplet at 5.299 ppm) to H-7 (aromatic hydrogen at

7.217 ppm) and of H-3′ (methyl singlet at 3.881 ppm) to

H-4′ (aromatic hydrogen at 7.912 ppm).

1

determined by H NMR integration of diagnostic proton sig-

nals of products relative to the area of peak due to internal

standard tetrachloroethane (δ 6.3, s, 2H).

Photolysis products of 3

2,3-Dimethybenzothiazolium (82%, 8.25 (d, J = 8.2 Hz,

H-7), 8.09 (d, J = 8.5 Hz, H-4), 7.92 (t, J = 7.2 and 8.5 Hz,

H-5), 7.83 (t, J = 7.2 and 8.2 Hz, H-6), 4.16 (s, 3H), 3.10 (s,

3H)), phenanthridine, 1,1,2,2,-tetraphenylethane (93%),

adduct 7 (27%), CH₃OH(D) (54%).

Photolysis products of 4

2,3-Dimethybenzothiazolium (87%), phenanthridine (85%),

2,3-dimethyl-2,3-diphenylbutane (70%), α-methylstyrene

(5%), cumene (18%), adduct 7 (13%), CH₃OH(D) (63%).

Photolysis products of 5

2,3-Dimethybenzothiazolium (88%), phenanthridine (83%),

bis-(9-methyl-9-fluorenyl) (9) ((85%) δ: 7.49 (d, J = 7.5 Hz,

4H), 7.23 (t, average J = 7.4 Hz, 4H), 7.06 (t, average J =

7.4 Hz, 4H), 6.88 (very broad, 4H), 1.91 (s, 6H)), 9-methyl-

fluorene (11%), adduct 7 (13%), CH₃OH(D) (66%).

Shukla et al.

757

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Financial support was provided by the National Science

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